In the heavy, moist air of the Amazon rainforest, I sit waiting as an old shaman pours an ancient sacrament into a cup. The brew he has prepared is ayahuasca, a blend of two plants that provides a visionary experience of such sublime, boundary-dissolving beauty that it changes the way you see the world for ever. The shaman is participating in humanity's oldest form of spiritual practice. Not only does the use of visionary plants predate organised religion by tens of thousands of years, but many anthropologists believe that the presence of hallucinogens in the diet of our hunter-gatherer ancestors had a significant influence on the way our brains evolved. Millions of people, me included, use these substances for spiritual growth, metaphysical exploration and healing.

However, shamanism cannot be described as a religion or a faith. No faith is needed in a visionary experience; in these states, the individual receives direct personal experience of the divine, becoming unified with their own subconscious and with the rest of the universe. In a timeless moment you realise that God is not an angry patriarch somewhere in the ether – God is within. We are the arbiters of good and evil, entirely responsible for creating our own reality. This ecstatic realisation cannot be enshrined in dogma, requires no priests and does not ask one to have faith in the ancient ideas of other people. It is no surprise that hallucinogenic plants and chemicals are also known as "entheogens", a word derived from Greek that means "that which generates the god within".

Entheogens are illegal in most countries, but the same societies that condemn entheogens actively promote the use of alcohol, a drug that – according to a study by the Centre for Addiction and Mental Health – may be responsible for 1 in 25 deaths worldwide. Plant medicines are incredibly safe by comparison and inspire peaceful and productive behaviour, which suggests that drug laws are based more on cultural conditioning and preconceptions than on reason.

The legality of alcohol and cigarettes indicates that the danger of a drug is not the primary factor in deciding its legality. What matters is that the drug does not interfere with the dominant cultural ideology of a society. Entheogens destroy an individual's cultural conditioning, freeing them from a fixed perceptual framework and encouraging them to think independently. Western cultures cannot incorporate experiences like this into their cultural framework because to do so would be to risk a serious transformation of culture itself. One only has to look at the effect that mass use of LSD had in undermining the moral assumptions of the US in the late 1960s to see why governments are terrified of these substances.

The tragedy of prohibition is that entheogens have the potential to be the most successful psychiatric medicines known to man. Fortunately, the medical community and some governments are beginning to recognise this, and there has been a resurgence in psychedelic research in the last five years. Organisations like the Multidisciplinary Association for Psychedelic Studies in the US have studied the use of MDMA, psilocybin and other psychedelics for a range of illnesses and conditions, including post-traumatic stress disorder and drug addiction.

As someone who uses psychedelics as a spiritual technology, I am not surprised by the very promising results of these studies. My first psychedelic experience completely changed my life and convinced me that the use of hallucinogenic plants is a human birthright. To find spiritual peace in this way and be told by your society that you were wrong for seeking it is saddening and frustrating. No one has the right to tell another person how they can experience the divine. Freedom of religion is an inalienable right, and until this right is extended to the oldest form of spiritual practice, our ability to explore who we truly are will be severely limited.